Bluetooth Home Automation: Control 4 Appliances from Your Phone

Switch four lamps or appliances from any Android phone using a free serial Bluetooth app, an HC-05 module and a 4-channel relay board.

What you’ll build

The HC-05 gives your Arduino a Bluetooth serial link. Single-character commands from a phone app (“1”/“a” toggles channel 1, and so on) flip the corresponding relay on the 4-channel board, each of which can switch a mains appliance through its NO contact.

Components you’ll need

Step-by-step guide

1. Wire the HC-05: VCC to 5V, GND to GND, TX to Uno D10 and RX to D11 through a simple divider (1K into the pin, 2×1K to GND) since the module is 3.3V-logic.
2. Connect the relay board: VCC to 5V, GND to GND, IN1–IN4 to D4–D7.
3. Upload the sketch, then pair your phone with the HC-05 (PIN 1234).
4. Install any “Bluetooth serial controller” app, connect to HC-05, and assign buttons that send 1/2/3/4 for ON and a/b/c/d for OFF.
5. Test with lamps: wire each lamp’s live wire through a relay’s COM and NO terminals. Only work on mains wiring if you are qualified — 230V surprises are permanent.

The code

SoftwareSerial keeps the hardware serial port free for debugging. Each received character maps to one relay.

#include <SoftwareSerial.h>
SoftwareSerial bt(10, 11); // RX, TX
const int relays[4] = {4,5,6,7};
void setup(){
  bt.begin(9600);
  for(int i=0;i<4;i++){ pinMode(relays[i],OUTPUT); digitalWrite(relays[i],HIGH); }
}
void loop(){
  if(bt.available()){
    char c = bt.read();
    if(c>='1' && c<='4') digitalWrite(relays[c-'1'], LOW);   // ON
    if(c>='a' && c<='d') digitalWrite(relays[c-'a'], HIGH);  // OFF
  }
}